For the continuous production of plaster sheets, especially plasterboard, gypsum (calcium sulfate hemihydrate) is admixed with water in a plaster mixing device to form a plaster paste (mortar), which is deposited onto a continuous moving endless belt and there allowed to setup (harden). Finally, the set plaster (calcium sulfate dihydrate) is cut into sheets.
Plaster mixing devices are described in U.S. Pat. Nos. 1,758,200 and 2,253,059 wherein a rotating disk or both the rotating disk and the underside of the housing cover have rows of pins arranged radially or spirally, where the rows of pins on the rotating disk and the housing cover penetrate into one another.
Water, gypsum (plaster of Paris) powder, and additives are fed from above onto such a rotating disk. By means of centrifugal force, the solid and liquid materials are propelled outward between the pins. They are thereby mixed, and in some cases further mixed by an arrangement of coarse teeth on the rotating disk. The plaster paste flows past the edge and down through any teeth which may be present, then through one or more outlets to the conveyer belt.
A disadvantage of such mixers is that as new material is continually added, plaster paste intrudes under the rotating disk into the clearance space between the disk and the housing floor and then sets up. The hardened deposits of gypsum dihydrate which are thus formed can jam or bind the rotating disk. They become broken down and cause abrasion. The broken down dihydrate fragments are carried along with the plaster paste to the outlet of the mixer and onto the conveyer belt.
A further disadvantage of such mixers is that the rows of pins create stagnant zones, in which the plaster paste can be trapped and then sets up. In this way, dihydrate deposits form, which break off and get broken down further to small particles which also get delivered to the conveyer belt along with the plaster paste.
The dihydrate particles in the plaster paste tend to form stable clusters of unbindable dihydrate resulting in local acceleration of the setting of the plaster in these regions. This leads to irregularly distributed soft and hard places in the finished plasterboard. Since the rate of production is predicated on a constant setting rate, uneven acceleration also causes production disturbances.
A further disadvantage of these pin mixers is that certain grades of plasterboard can generally not be made, or can be made only with difficulty, because the interpenetrating pins hinder the admixture of foam, chips, long glass fibers, or other fibrous materials to the mortar.
Foam which is added is broken down by the interpenetrating pins, forming large bubbles and resulting in inhomogeneous texture, therefore to density variations in the finished sheets.
Chips which are added become lodged between the pins. Long fibers wrap around the pins and form skeins. Both effects disturb the operation of the machine, influence the quality of the plaster mix, and lead to heavy maintenance requirements.
Even without addition of chips or fibers, the pins are subject to constant abrasion, so that they often have to be replaced.
An even more fundamental disadvantage of the pin mixer is that dihydrate particles and in some cases, chips and fibers, get above and beneath the rotating disk, and cause a major requirement for repair and maintenance by obstructing the mixer, by blocking or jamming the rotating disk, and by abrasive action.
German Patent Application No. 29 31 782 describes a category of mixers wherein the pins are replaced by interpenetrating deflecting blades and circulating pump elements with resultant improvement in mixer efficiency. Also, a scraper is provided on the underside of the rotating disk, which extends into the small clearance space and serves to remove any intruding plaster paste.
The scraper still has the disadvantage that it abrades so that the distance between the rotating disk and the floor of the housing must be periodically readjusted. Precise readjustment is impossible, because it is impossible to see into the clearance space.
Besides the maintenance by means of readjustment, it is characteristic of such a scraper that because of the difficulty of readjustment, it is not possible to completely prevent the plaster paste from intruding under the rotating disk and forming dihydrate particles in that region.
Although the deflecting blades and the circulating pump elements are designed and arranged so that the stagnant flow regions are reduced as compared to the pin mixer, nevertheless it is impossible to completely forestall the formation of dihydrate particles which get into the plaster paste and which strongly affect the quality and production of the sheets.
These interpenetrating deflecting blades and circulating pump elements also break down any foam which may be added and they form barriers for chips and long fibers. Thus with these mixers also, there are certain grades of plasterboard which either cannot be made or can be made only with difficulty.
Similarly with these mixers there is a high maintenance requirement because of the wear on the deflecting blades and on the circulating pump elements.
In a further pin mixer, described in German Patent Application No. 36 11 048, coarse admixed materials which occur in plaster paste and which intrude into the crevice between the underside of the rotating disk and the floor of the housing, no longer can jam the rotating disk. In this type of plaster mixer, at the forward edge of teeth attached to the rotating disk, there is a cutting edge which rides close to the floor of the housing, with a space behind the cutting edge. Granular or fibrous additives which intrude are repelled radially outward by the wiping edges of wiping strips located on the underside of the rotating disk. This process is enhanced by the jet action of a water and an air stream conducted through the housing body.
Even in this mixer, gypsum pastes containing coarse additives succeed in intruding under the rotating disk. Therefore, the protective laminate and the stripping bars become worn and have to be replaced. It is also disadvantageous that because of the blower-like action of the stripping bars, air is pressured into the plaster paste.
The formation as well as the discharge of dihydrate particles onto the conveyer belt with adverse effects on the quality and production of the plasterboard are not prevented; the plaster paste below and especially above the rotating disk, which is provided with stirring blades, can set up to dihydrate. The production of those grades of plasterboard which require foam, chips or fibers is made more difficult.